A TiCN thin film, which has been widely used as a hard coating layer for cemented carbide cutting tools, is formed through a reaction at a high temperature of about 1000° C. by using reaction gases such as TiCl4, CH4, and N2. During this process, carbon (C) is diffused from a cemented carbide parent material into the TiCN thin film to form a very hard and brittle phase, such as Co3W3C or Co6W6C, at an interface between the parent material and the TiCN thin film, and thus, toughness of a cutting tool may decrease.
In order to address the foregoing limitation, a method of forming a TiCN thin film by using a moderate temperature chemical vapor deposition (hereinafter, referred to as “MTCVD”) is suggested. This method uses TiCl4 and CH3CN as a source of carbon and nitrogen (N) during the formation of TiCN to decrease a deposition temperature to a range of about 750° C. to 850° C., and thus, generation of the very hard and brittle phase, such as Co3W3C or Co6W6C, is controlled by inhibiting diffusion of carbon from the cemented carbide parent material to TiCN during the formation of the TiCN thin film. Thus, the method is characterized by that the TiCN thin film formed after coating has high toughness as well as wear resistance.
The foregoing MTCVD TiCN thin film is commercialized in a multilayered structure, in which a bonding layer is formed on the MTCVD TiCN thin film and then oxide, such as an alumina layer, is formed thereon, and recently, used widely in cutting tools for turning and milling.
Heat as well as wear of a thin film is generated due to the friction between a cutting tool (insert) and a workpiece during machining, and in a multilayered thin film having a stack structure in the sequence of a MTCVD TiCN layer, a bonding layer, an alumina layer (Al2O3) from a parent material, a cover layer is additionally formed on the alumina layer, an uppermost layer, in order to identify the usage of a cutting tool or distinguish from products of other makers.
However, with respect to a stress state of the foregoing typical chemical vapor deposition (CVD) coating layer for cutting tools, since surface residual stress is maintained in a tensile stress state up to the alumina layer as shown in FIG. 1, the wear-resistant coating layer may be easily damaged due to the effect of residual stress, and thus, lifetime of the coating layer may be decreased during machining.
In relation to the foregoing, methods have been typically attempted, in which compressive residual stress is applied to a surface of a coating layer by using a separate process, such as a blasting treatment, or a compressive residual stress state is generated by coating a cover layer by using a physical vapor deposition (PVD) method after forming an alumina layer or the like by using a CVD method.
However, a separate process may be added due to the blasting process, and a process of mixed use of the CVD and PVD method may not only be inconvenience but adhesion between the CVD layer and the PVD layer may also be poor, and thus, physical properties of the coating layer may deteriorate.